The cortex is an outer layer of the brain that is bounded by an outer boundary and an inner boundary. The outer boundary is the convoluted grey surface familiar from many depictions of the brain. The inner boundary is a surface toward which many white matter tracks are directed. These tracks tend to end at the inner boundary.
The direction that extends across the cortex, from the inner boundary to the outer boundary will be referred to as the radial direction of the cortex. Since the cortex is not spherical, the notion of a radial direction is one that is locally defined. The direction of the plane perpendicular to the radial direction, which is also locally defined, will be referred to as the transverse direction of the cortex.
In an effort to understand the structure of the cortex, it is useful to measure its diffusivity. Known methods of measuring diffusivity include those in which one obtains MRI images of the brain.
Diffusivity is a vector quantity having an associated direction. Within the cortex, the diffusivity is dominated by its radial component. However, within the cortex, there appears to be little observable structure in the radial component of diffusivity. What structure there is appears to be more readily elucidated by examining the transverse component, not the radial component. Unfortunately, the transverse components of diffusivity are precisely the components that are the most difficult to discern.